One of the many reasons why it’s hard to shrink the size of government is that even when a program has ran its course, or when it’s not needed anymore, the interest groups that benefit from it will fight to keep it going, and Congress is happy to side with special interests at the expenses of taxpayers.

This is what’s happening right now with the Defense Department and tanks. On one hand, the army is basically saying, “Tanks were great but we don’t need any more of them to fight modern wars, especially given the current budget environment,” and defense contractors and Congress are saying, “Well, we’ll see about that.” The Washington Postreports:

The manufacturing of tanks — powerful but cumbersome — is no longer essential, the military says. In modern warfare, forces must deploy quickly and “project power over great distances.” Submarines and long-range bombers are needed. Weapons such as drones — nimble and tactical — are the future.

Tanks are something of a relic.

The Army has about 5,000 of them sitting idle or awaiting an upgrade. For the BAE Systems employees in York, keeping the armored vehicle in service means keeping a job. And jobs, after all, are what their representatives in Congress are working to protect in their home districts.

The Army is just one party to this decision. While the military sets its strategic priorities, it’s Congress that allocates money for any purchases. And the defense industry, which ultimately produces the weapons, seeks to influence both the military and Congress.

Here’s how that works:

Military officials say they’ve given careful thought to their strategy and they simply can’t afford to pay for more upgraded tanks.

Gen. Raymond Odierno, the Army’s chief of staff, made its case before Congress in 2012.

“We don’t need the tanks,” he said. “Our tank fleet is 21/2 years old average now. We’re in good shape, and these are additional tanks that we don’t need.” . . .

But the Army has run up against congressional opposition. To keep these lines running, Congress has allocated well more than the Army requested for the programs — an extra $181 million for Abrams in fiscal 2013 and about $140 million more for Bradley.

Legislators say they don’t want the money they’ve invested in building up the country’s vehicle-making capability to go to waste. The several hundred million dollars it would cost seems to them a small amount relative to the billions spent on defense annually.

The industry, too, has pushed Congress to support its work. Last year, BAE convened its suppliers — it has 586 across 44 states — in Washington to storm the Hill, chatting up representatives about the jobs they provide and pushing for Congress to help the Bradley program.

The whole story is here. And it’s not always Congress and the defense industry — sometimes it’s the White House that pushes back against defense cuts that the Pentagon proposes.

In a paper called “The Pathology of Privilege: The Economic Consequences of Government Favoritism,” my colleague Matt Mitchell explains how these special-interest politics work. ”Whatever its guise, government-granted privilege [to private businesses] is an extraordinarily destructive force. It misdirects resources, impedes genuine economic progress, breeds corruption, and undermines the legitimacy of both the government and the private sector,” he writes.

The fact that defense is a totally legitimate function of government doesn’t spare it from the same forces of cronyism that affect other government-entangled industries.